1. Field of the Invention
The invention relates to the field of telemetry used in biomedical applications and in particular to telemetry of digital biomedical data, including in particular digital telemetry of cardiac signals.
2. Description of the Prior Art
Telemetry systems that transmit a plurality of patient biosignals, such as ECG signals, are well known. A number of patients, such as may be cared for in an intensive care unit will each be coupled to a transmitter at the patient's location. The telemetry unit transmits to a central nurses' station, which monitors a plurality of patient transmitters through a multiplexing arrangement. Any biomedical signal may be telemetered, but cardiac signals or ECG signals, blood pressure, respiration rates, pulse rates and other vital signs are typically monitored through conventional means and transmitted to the nurses' monitoring station. Personnel may thereby continuously monitor the vital signs of a large number of patients. In addition, one or more of the vital signs for each of the patients may be coupled to a software driven alarm in the event that an abnormality is detected.
The analog data which is transmitted is often susceptible to large DC biases picked up by the ECG leads of other biomedical instrumentation leads and transmitted as true data. In addition, depending on the bit patterns, prior digital data would have intervals of high ones density or high zeroes density which in frequency modulated data tend to cause a average droop or tilt transmitted as a DC drift.
This DC drift not only plays havoc with alarm settings in the monitoring station, but also requires additional dynamic range or bandwidth and hence has effect of reducing the sensitivity of the receiving circuitry at the monitoring station. In addition, frequency modulated analog data typically has to be converted to digital data for more sophisticated and varied processing in the monitoring station, for example, for the recognition of a particular type of cardiographic pattern, which may be indicative of a heart behavior more subtle than cardiac arrest.
Therefore, what is needed is an apparatus and methodology for telemetering the biomedical data in digital form in a manner such that the biases introduced through the ECG leads and other medical probes do not corrupt the data or require complex electronic or software compensations.